Digestion of lignocellulosic material with an organomercaptan and an inorganic digesting aid



United States Patent 3,490,991 DIGESTION OF LIGNOCELLULOSIC MATERIAL WITH AN ORGANOMERCAPTAN AND AN IN- ORGANIC DIGESTING AID William E. Fisher, Waterville, and Shibley A. Hider, Toledo, Ohio, assignors to Owens-Illinois, Inc., a corporation of Ohio No Drawing. Filed Dec. 30, 1966, Ser. No. 606,025 Int. Cl. D21c 3/04, 3/02, 3/20 U.S. Cl. 162-76 17 Claims ABSTRACT OF THE DISCLOSURE Methods of pulping lignocellulosic material using a two-stage treatment which includes the steps of (A) digesting the lignocellulosic material with a treating liquor containing (a) at least one organomercaptan such as thioglycolic acid and (b) at least one inorganic digesting aid such as sodium hydrosulfide, the treating liquor having an initial alkaline pH not exceeding about 12.0, the time and temperature of the digestion being sufiicient to convert the lignocellulosic materials to a treated material containing mercaptan-reacted lignin; and (B) extracting the mercaptan-reacted lignin retained by the digested material by contacting it with a dilute solution of a water-soluble inorganic base such as sodium hydroxide.

This invention relates broadly to technique for treating (more particularly delignifying) lignocellulosic material including both softwoods and hardwoods. It is especially concerned with a treatment leading to the pulping of such material and which involves two main stages or steps: (a) a digestion or cooking step and (b) an extraction step. (For convenience and brevity such a treatment is sometimes referred to hereafter and in the appended claims as a two-stage treatment.)

Still more particularly the invention is concerned with the treatment of lignocellulosic materials by first digesting such material with a treating liquor containing both (a) an organic thio compound, especially an organomercaptan, e.g., thioglycolic acid (TGA),

and (b) an inorganic digestion agent or aid. The latter includes, for example, the alkali-metal hydrosulfides, monosulfides, polysulfides, sulfites, bisulfites and borohydrides such as sodium hydrosulfide (NaSH), sodium monosulfide (Na S), sodium polysulfide (Na s sodium sulfite (Na SO sodium bisulfite (NaHSO sodium borohydride (NaBH and others known in the art.

The preferred inorganic digestion agent that is used in conjunction with the organic thio compound is an alkali-metal sulfide, more particularly sodium monosulfide and/or sodium hydrosulfide. The alkali-metal sulfides, specifically Na S, NaSH and mixtures thereof, are not the full equivalent of the other inorganic digestion agents in this particular environment, and especially when the organic thio compound is thioglycolic acid.

The treating liquor employed in the digestion step is characterized by having an initial pH within the range of from above 7 to 13, and preferably is adjusted so that the initial pH ranges from 9.0 to 12.0. The initial pH decreases as cooking is continued, and may dop to a pH below 7.0. However, it is preferred that the final pH of 3,490,991 Patented Jan. 20, 1970 the treating liquor at the end of the digestion period be above 7.0.

The digestion step provides a digested or cooked material containing mercaptan-reacted lignin. This mercaptan-reacted lignin is subsequently extracted from the cooked material by contacting it with a dilute solution of a soluble (preferably water-soluble) inorganic base, e.g., a dilute aqueous solution of sodium hydroxide.

It was known prior to the present invention to digest or cook wood, specifically spruce sawdust, with an organomercaptan, more particularly TGA. See, for example, Ingeniors Vetenskaps Akadernien, Proceeding No. 103, pp. (1930), The Mercaptans of Pinewood, by Bror Holmberg. Holmbergs procedure was to treat, for instance, spruce sawdust with a solution containing TGA and which had been made strongly acidic with hydrochloric acid. In a second step the mercaptan-reacted lignin was extracted by treating the digested Wood with an aqueous solution of caustic soda.

Currently lignooellulosic material, specifically pinewood, is delignified or pulped" (i.e., digested or cooked) by the Kraft process (yield, about 52-55%) to obtain paper and other finished cellulosic products having optimum tensile and tear properties. If cooking conditions are changed to obtain higher yields of pulp, the properties of the resulting pulp are poor.

Surprisingly and unobviously it was found that, by practicing the present invention, one can obtain a higher yield of pulp (and of paper made from the pulp) as compared with the results obtained from a kraft cook; and, also, with less alkali than is required by the kraft process. Furthermore, by practicing the present invention, papers can be made that have improved strength properties, especially in tear values, as compared with the results obtained when cooks are made with acidic treating liquors (pH of, for example, from 1 to 6) containing TGA alone as the digestion or delignification agent.

Another unexpected advantage that is secured when the instant invention is practiced is that the mercaptoreacted lignins have greater alkaline solubility and higher sulfur content than do the lignins resulting from the kraft process; and therefore, the mercapto-reacted lignins resulting from the instant invention can be readily removed from the digested lignocellulosic material by treatment with a dilute solution of a water-soluble inorganic base.

It was also found that one could incorporate up to about 50% by weight of hardwoods into mixtures thereof with softwoods, and obtain high yields of pulp and from which papers could be made having good strength properties.

The foregoing discoveries are of considerable economic importance and significance. The economic and marketing advantages accruing from the obtainment of pulps in higher yields and of papers therefrom of good properties, as compared with the results provided by the kraft process, will be apparent to those skilled in the art.

In practicing the present invention any wood or other lignocellulosic material, or mixtures thereof in any proportions, may be cooked or digested with a treating liquor containing both an organomercaptan and an inorganic digestion agent, and having a controlled alkaline pH within the aforementioned ranges. The treating liquor is brought into intimate contact with the lignocellulosic material either with or without first removing the extractives by treating the lignocellulosic material in sub-divided form (e.g., in the form of sawdust, shavings, wafers, and/ or chips) with an organic solvent capable of extracting the organic solvent-soluble components of the material. Such lignocellulosic materials include softwoods, hardwoods and fibrous annual plants.

Examples of softwoods are balsam, fir, eastern hemlock, jack pine, eastern white pine, red pine, black spruce, red spruce, white spruce, tamarack and cypress. Examples of hardwoods are black gum, quaking aspen, mixed tomahawk, American beech, paper birch, yellow birch, eastern cottonwood, sugar maple, silver maple; yellow poplar, black cherry and white oak. Examples of fibrous annual plants are bagasse, hemp and jute. Mixtures of woods or other lignocellulosic materials of different origin may be used if desired; e.g., mixtures of different so-ft woods, or of different hardwoods, or of one or more softwoods and one or more hardwods.

THE ORGANOMERCAPTAN REACTANT Illustrative examples of organomercaptans that may be employed in digesting wood or other lignocellulosic material in practicing this invention are those embraced by the general formula wherein R represents a divalent radical, more particularly a divalent hydrocarbon radical, Y represents a monovalent substituent bonded directly to R, and n represents a numeral ranging from up to the combining power (i.e., a value that will completely satisfy all valences) of the divalent radical represented by R.

Illustrative examples of divalent radicals represented by R in Formula I are divalent hydrocarbon radicals and, more particularly, divalent aliphatic, especially divalent saturated aliphatic, e.g., ethylene, propylene (trimethylene), butylene, isobutylene, pentylene, isopentylene, decylene, etc., including divalent cycloaliphatic, especially divalent saturated cycloaliphatic, e.g., cyclopentylene, cyclohexylene, cycloheptylene, etc.; divalent aromatic, e.g., phenylene, naphthylene, etc.; divalent aliphatic-substituted aromatic, e.g., 2,4-tolylene, ethyl-2,5- phenylene, isopropyl-3,4-phenylene, 1-butyl-2,4-naphthylene, etc.; divalent aromatic-substituted aliphatic, e.g., phenylethylene, phenylpropylene, naphthylisobutylene, xylylene, etc.; and radicals that may be classed either as divalent aromatic-substituted aliphatic or divalent aliphatic-substituted aromatic, e.g., 4,alpha-tolylene, 3,betaphenyleneethyl, 4,alpha-xylylene, 2,gamma-phenylenebutyl, etc. Thus R may represent a divalent hydrocarbon radical represented by the general formula wherein Ar represents an arylene radical and R represents an alkylene radical. Preferably the divalent hydrocarbon radical representcd by R contains not more than carbon atoms, more particularly from 1 to 8 carbon atoms.

Preferably, also, the divalent radical represented by R in Formula I is free from olefinic or acetylenic unsaturation either in a straight chain or in a side chain.

It is not essential that the divalent radical represented by R be composed solely of carbon and hydrogen atoms. For example, the chain of carbon atoms, whether straightchain aliphatic or carbocyclic, may be interrupted in the chain by other atoms, e.g., by oxygen and/ or sulfur and/ or nitrogen atoms bonded directly to carbon atoms of the chain.

Illustrative examples of substituents represented by Y in Formula I are functional groups such as OH; CN; SH; COOH; COOR', wherein R is a monovalent hydrocarbon radical corresponding to the divalent hydrocarbon radicals represented by R in Formula II; COOM, wherein M is a salt-forming cation, e.g., NH or Na, K, Li or other alkali metal, a salt-forming amine such as men dior tri-(hy o ar on-su s i 4 or -(hydroxyhydrocarbon-substituted) amine, or other salt-forming cation and especially those which yield water-soluble salts when present in the particular thio compound. Or, Y may be a radical represented by wherein R" and R' are members of the group consisting of hydrogen and monovalent hydrocarbon radicals corresponding to the divalent hydrocarbon radicals represented by R in Formula I.

It will be understood, of course, by those skilled in the art that when n in Formula I represents zero (0), then there are no radicals represented by Y in the formula, which latter then becomes (III) PIS-R wherein R represents a monovalent radical, more particularly a monovalent hydrocarbon radical corresponding to the divalent hydrocarbon radicals represented by R in Formula I. Illustrative examples of mercapto compounds embraced by Formula III are the alkyl (including cycloalkyl), aralkyl, aryl, and alkaryl mercaptans, more particularly those which contain from 1 through 10 carbon atoms and especially those having not more than about 8 carbon atoms.

The relatively low water-solubility of the unsubstituted hydrocarbyl mercaptans embraced by Formula HI makes them much less suitable for use than substituted hydrocarbyl mercaptans having one or more polar or solvating substituent groups. However, if water-solubility of the mercapto reactant is unimportant, e.g., when it is to be used in undiluted state, or in solution in an organic solvent (e.g., ethanol) or in a mixture of water and an organic solvent in which mixture the unsubstituted hydrocarbyl mercaptan is adequately soluble, then a mercaptan within the scope of Formula III may be employed as the treating agent.

Particularly useful in practicing the present invention are organomercaptans represented by the general formula wherein Z represents an alkylene (including cycloalkylene) radical containing from 1 through 10, and preferably from 1 through about 8, carbon atoms; R represents a member of the group consisting of (a) hydrogen, (b) alkyl radicals containing not more than about 10 carbon atoms and preferably a lower alkyl radical (e.g., an alkyl radical containing from 1 through about 6 carbon atoms); and (c) a salt-forming cation, examples of which have been given hereinbefore with reference to M in the grouping COOM which may be a substituent represented by Y in Formula I; and n represents an integer from 1 up to that of the combining power of the alkylene radical represented by Z. The alkylene radical represented by Z may be straight-chain, branched-chain or cyclic as in, for example, cyclopentyl, cyclohexyl and the like.

More specific examples of mercapto compounds embraced by Formula IV are monocarboxylic and polycarboxylic acids such as those having the formulas v ns-om-om-oo OH vr rrs-orr-om-oo OH v11 rrs-orr-co on vrrr 011920-43 0 OH SIH 00011 x Hs-on-ooon CHZ-COQI'I the ammonium, alkali-metal (sodium, potassium, lithium, etc.) and other water-soluble salts of the aforementioned monoand dicarboxylic acids; and the cyclopentyl and cyclohexyl esters, as well as the methyl, ethyl and propyl through pentyl( normal or isomeric alkyl) esters of the aforesaid acids. In the case of the salts and esters of the dicarboxylic acids, one may use either the monoor di-salt, or a mixture thereof, or a monoor diester, or a mixture thereof.

As the organomercaptan reactant we prefer to use thio acids, or salts or esters thereof represented by the general formula wherein n represents an integer from 1 to 6, inclusive, more particularly from 1 to 4, inclusive, and R has the same meaning as given above with reference to Formula IV. Thus, compounds embraced by Formula XI may be the thio acid itself or a salt (especially a water-soluble salt) or an ester of such an acid. Of these compounds thioglycolic acid and the water-soluble salts and the lower alkyl esters thereof are the more preferred sub-group. Mixtures of acids and/or salts and/or esters embraced by Formula XI may be used if desired.

Instead of employing organomercaptan compounds that are within the scope of Formula XI, one may use those wherein the COOR group in that formula has been replaced by other hydrolyzable or solvating groups such as -OH, -CN, -SH,

and

0 R H ON wherein R" and R in the last two groups are hydrogen or a monovalent hydrocarbon radical corresponding to one of the divalent hydrocarbon radicals represented by R in Formula I.

THE INORGANIC DIGESTION AGENT As was mentioned hereinbefore the inorganic digestlon agent that is used, in combination with an organomercaptan, as an essential component of the treating liquor is preferably sodium hydrosulfide or sodium monosulfide. Sodium polysulfide or other inorganic digestion agents (numerous examples of which previously have been given), or mixtures thereof in any proportions, may be used in lieu of all or part of the preferred inorganic digestion agents, NaSH and/or Na S.

The inorganic digestion aid, specifically NaSH, constitutes a minor amount (less than 50 weight percent) of the total amount of organic mercaptan and NaSH or other inorganic digestion agent. Preferably the inorganic modifier of the cooking liquor, e.g., NaSH, constitutes from about to less than 50 (e.g., to 49.9) weight percent, and still more preferably from about or to about or weight percent, of the total amount of the organomercaptan and inorganic digestion agent. In all preferred embodiments of the invention the organomercaptan constitutes a major amount by weight and the inorganic digestion aid constitutes a minor amount by weight of the total amount thereof in the treating liquor.

The minimum amount of inorganic digestion aid employed is that amount which will impart improved action to a treating liquor for lignocellulosic material that contains an organomercaptan as compared with a treating liquor containing an organomercaptan, specifically TGA, and no inorganic digestion agent. In some cases, it is contemplated that a detectable, or even an appreciable benefit may be observed when the inorganic digestion agent constitutes as little as 2 or 3 weight percent of the total amount of organomercaptan and inorganic digestion aid. For example, such small amounts, or amounts up to 10 or 15 weight percent of the total organomercaptan and inorganic cooking agents employed, may be particularly desirable when it is sought to produce papers from the resulting pulps that have maximum brightness values.

The use of NaSH or Na S or other inorganic cooking agent in an amount corresponding to 50% by weight and higher (e.g., from 50 to 75% or even up to or more), based on the total organomercaptan and inorganic digestion aids used, is not precluded. However, no particular advantages appear to accrue therefrom and there are certain disadvantages, e.g., an inability to produce a pulp from which can be made papers and paper-like products having a desired degree of brightness. Where this property is unimportant for a particular service application, then it is contemplated that one may find it economically advantageous from a raw materials cost standpoint to use the higher amounts, just mentioned, of the inorganic digestion agent.

THE SOLUBLE INORGANIC BASE Any soluble, more particularly water-soluble, inorganic base (including salts and other substances that are hydrolyzable to an inorganic base) may be employed (singly or a plurality thereof) as an extractant of the mercaptan-reacted lignin from the lignocellulosic material that has been digested with a treating liquor wherein the active or effective treating agent comprises or consists essentially of (a) an organomercaptan and (b) an inorganic digestion agent, specifically sodium monosulfide or sodium hydrosulfide. By inorganic base is meant any inorganic compound or substance that, at some concentration, will impart alkalinity (i.e., a pH above 7.0) to a solution thereof in water at 25 C. By water-soluble base is meant any inorganic base that has at least some solubility in water.

Illustrative examples of suitable inorganic bases include the alkali-metal (especially sodium and potassium, but preferably sodium for economical reasons) hydroxides and carbonates (including the bicarbonates); the alkalimetal phosphates, silicates and borates, e.g., sodium metasilicate, trisodium phosphate and the various sodium tetraborates including borax; alkali-metal salts of organic acids, e.g., sodium and potassium acetates; and others that will be apparent to those skilled in the art from the foregoing illustrative examples.

Preferably the inorganic base is an alkali-metal hydroxide or carbonate which is used in the form of a dilute aqueous solution. The concentration of the base in the solution is important, especially when a strong base such as sodium hydroxide is employed. In such a case and in order to minimize degradation of the cellulose in the digested lignocellulose, it is preferred to use a dilute aqueous solution of sodium hydroxide that contains from 0.5 to 5, more particularly from 0.8 to 2, weight percent of NaOH.

The aims, objects and purposes of this invention, including those indicated earlier in this specification, are attained by providing a method of modifying or altering (more particularly delignifying or so-called pulping) lignocellulosic material by a two-stage treatment which includes the combination of two essential steps A and B. By pulping it is meant that the lignocellulosic material is treated in such a manner that it is refinable to a papermaking or a dissolving grade of pulp.

In Step A the lignocellulosic material is cooked or digested with a treating liquor comprised of a reactive agent and by which term is meant a combination of two different types of materials: the first is one or more organic thio compounds (especially one or more organomercaptans, e.g., TGA), and the second is an inorganic digestion agent or aid, especially sodium monosulfide or sodium hydrosulfide. The relative proportions of these two components of the reactive agent have been given hereinbefore under the heading The Inorganic Digestion Agent.

One of the distinct advantages of using an organomercaptan in a treating liquor containing an alkali, as is the case with the treating liquors employed in practicing the present invention, is that the organomercaptan reacts with the lignin in the wood thereby making it more soluble in the alkaline liquor. Also, especially when a preferred organomercaptan of the type exemplified by TGA is employed, it functions as a pH control by reason of its strong buffering action. Thus, the organomercaptan component of the treating liquors with which this invention is concerned serves to decrease the loss of carbohydrates that normally results from degration by alkali and makes possible higher pulp yields.

The reactive agent (e.g. TGA plus NaSH) is present in the treating loquor in an amount corresponding to at least 2.5, preferably at least 5, weight percent based on the weight of the oven-dried (O.D.) lignocellulosic material. The pH of the treating liquor is generally adjusted so that the initial pH does not exceed about 12.0, thus providing for the desired two-stage treatment and thereby minimizing hydrolytic attack of the alkaline treating liquor upon the cellulose molecule in the lignocellulosic material during the digestion treatment.

(Parenthetically it may here be mentioned that the higher initial pH up to 13 to which reference previously has been made is contemplated, e.g., a pH of 12.5-13.0, in the treatment of more highly acidic (i.e., above normal) lignocellulosic materials. In such cases the higher initial alkalinity above pH 12.0 serves to neutralize the acidity in the lignocellulosic material that is above the normal or usual amount in the average lignocellulosic material.)

The temperature and time of digestion of the lignocellulosic material in the treating liquor are sufiicient to convert the said material to 3. treated material containing mercaptan-reacted lignin.

As previously has been indicated, in Step B of the twostage process with which this invention is concerned, the mercaptan-reacted lignin retained by the digested lignocellulosic material is extracted by contacting it with a dilute solution of a soluble (preferably water-soluble) inorganic base, numerous examples of which have been given hereinbefore.

For economical reasons and to simplify the recovery of organomercaptan for re-use in the process, the method generally includes the step of removing the excess treating liquor from the treated material from Step A, after which the residue that remains is preferably washed with a washing fluid, e.g., hot water, before the extraction step described in Step B is carried out.

THE DIGESTION STEP In practicing the present invention, the digestion of the lignocellulosic material with the treating liquor (white liquor) may be effected, as previously has been mentioned, with or without first removing the extractives including tall oil therefrom. Advantageously, the extractives are first removed by pre-extracting the wood or other lignocellulosic material with an organic solvent, preferably isopropanol, l-butanol or other high-boiling organic solvent. If a hot fluid medium, e.g., steam, is used to remove the aforesaid organic solvent, it is desirable that the wood in subdivided (e.g., chip) form remain in swollen state before digestion in the cooking liquor in order to facilitate diffusion of the liquor into the chips and thorough impregnation of the latter.

The type of reaction vessel required for the cook depends upon such influencing factors as, for example, the conditions of the cook, the properties of the reactive agent (especially the properties of the organomercaptan component thereof), and the vapor pressure and alkalinity of the cooking liquor.

In carrying out the treatment of the lignocellulosic material with the reactive agent, the lignocellulosic material, in a suitably subdivided form, is saturated and/or covered with, or suspended in, a liquor containing the reactive agent, e.g., TGA plus NaSH. It is then cooked or digested as hereafter more fully described.

The treating liquor used in practicing this invention contains the previously defined reactive agent and which comprises, or is composed, or consists essentially of at least two different types of treating agents, one of which is an organic thio compound such as an organomercaptan and the other of which is an inorganic digestion agent or aid as exemplified by, and which preferably is, sodium monosulfide or sodium hydrosulfide. This reactive agent is dissolved, at least under the reaction conditions of temperature, pressure, pH, etc., in a liquid solvent medium which may be water; an organic solvent (e.g., isopropanel, l-butanol, l-hexanol, ethylene glycol, dimethylsulfoxide, dimethylformamide, dioxane, pyridine, N,N-dimethylacetamide and other commercial available organic solvents); or mixtures of water and an organic solvent. The reaction medium is preferably an aqueous liquid reaction medium, specifically water. The organomercaptan is preferably thioglycolic acid (TGA) and/or a watersoluble salt thereof.

The concentration of the reactive agent in the aforesaid liquid reaction medium cannot be stated with precision since it is dependent upon so many different influencing factors including, for example, the kind of wood or other lignocellulosic material being digested; the nature of its subdivided form; the particular components of which the reactive agent is constituted and the ratio of said components to each other; the ratio between the reactive agent and the lignocellulose; the time, temperature and pressure of digestion; type of digester employed; and other influencing factors. Generally, however, the reactive' agent constitutes, by weight, from about 0.5% to about 25%, more particularly from about 1% to about 10%, of the weight of the liquid reaction medium.

The digestion of the lignocellulosic material in the treating liquor is initiated at a pH of the treating liquor above 7.0, preferably at least 7.6, at which latter value the pH of the liquor is distinctly basic as distinguished from neutral or approximately neutral. It has been stated earlier in this specification that the treating liquor preferably has an initial pH within the range of from 9 to 12 (about 9 to about 12). Within this 912 initial pH range, the more preferred range is from 9.0 to 11.0.

If desired, a conventional buffering agent or system may be added to the treating liquor as an aid in regulating the pH of the said liquor; that is, to keep the pH from dropping undesirably low during the digestion. It is also contemplated that by adding a buffer to the digestion liquor one would be able, advantageously, to carry out the digestion step with the liquor at an initial pH of close to but slightly above 7.0.

For alkalinity adjustment (specifically pH adjustment) of the initial treating liquor one may employ an alkalimetal hydroxide, preferably sodium hydroxide, or other suitable inorganic base. Numerous examples of such bases have been given hereinbefore with reference to the inorganic-base component of the liquid extractant used in the extraction step. Alkaline organic compounds also may be employed, if desired, for providing the desired initial pH of the treating liquor, e.g., the various amines, quaternary ammonium compounds and other nitrogenous bases, and preferably those having a boiling point or boiling range above the maximum digestion temperature under the particular pressure conditions prevailing during the digestion period.

The cooking or digestion of the lignocellulosic material with a treating liquor comprising a combination of an organomercaptan, e.g., TGA, and an inorganic digestion agent, e.g., NaSH, may be effected at a temperature within the range of, for instance, from C. (preferably at least about 95 100 C.) to 225 C., more particularly from about 120 C. to 200 C., and still more particularly at from about 150 C.190 C. The time of treatment varies, for example, from /2 to 4 or 5 hours but which sometimes may be 6 or 8 hours or more since there is a time-temperature relationship. A time of from 1 to 2 hours at a temperature of from 160 to 180 C. is preferred. Other influencing factors in addition to temperature that are pertient to the time of treatment are, for instance, the type and degree of sub-division of the lignocellulosic material being treated, the chosen reactive agent, the amount of the reactive agent with respect to the lignocellulosic material, the concentration of the reactive agent in the treating liquor, the type and size of digester, the type of product desired, and other factors.

The concentration of the reactive agent, e.g., TGA plus NaSH, in the treating liquor with respect to the OD. lignocellulosic material should be at least 2.5%, preferably at least 5%, by weight thereof. Thus it may be, for example, from about 5% to about 100%, more particularly from 5 to 10% to about 50% (specifically about 25%) by weight of the OD. lignocellulosic material charged to the digester. For economical reasons no more reactive agent with respect to the amount of D. lignocellulosic material, and especially no more organomercaptan with respect to the total amount of organomercaptan and inorganic digestion agent in the reactive agent, should be used than is necessary to obtain the optimum yield of the desired product with optimum properties at minimum cost of time and labor.

The liquor recovered from the initial digestion with the treating liquor containing the aforementioned reactive agent may contain a significant amount of unused cooking chemicals. This liquor may be recrycled in the process after making up for the spent chemicals; or the residual chemicals may be separated from the liquor by such methods as ion exchange, solvent extraction, distillation, dialysis, etc., for re-use in the process.

OTHER STEPS IN THE METHOD In the two-stage method of this invention the excess liquor is preferably removed (e.g., by draining) from the reactor or digestor at the end of the cooking period, and the residue is Washed, e.g., with water and, preferably, hot water.

The washed, subdivided, residual wood containing mercaptan-reacted lignin is then either transferred to an extraction vessel; or the digester in which the initial cooking was carried out may be used as the extraction vessel.

Mercaptan-reacted lignin retained by the residue (preferably the residue remaining after draining off the excess liquor; and, more preferably, the residue that remains after washing the drained residue) is contacted with a dilute solution of a soluble (preferably water-soluble) inorganic base. Examples of such bases, the concentration thereof in the solution, and additional discussions have been given hereinbefore under the heading THE SOLU- BLE INORGANIC BASE.

In using dilute solutions, more particularly dilute aqueous solutions, of a water-soluble inorganic base as an extractant of the mercaptan-reacted lignin, the extraction temperature may range, for instance, from ambient temperature (about 2030 C.) to about 200 C., more particularly from about 50 or 100 C. to about 180 C. The extraction time may range from about A to about 4 or hours or longer, as desired or as conditions may require. Thus the extraction temperature and time may be 150 C.-170 C. for from 1 to 2 hours, more particularly when the extractant is a dilute aqueous solution of sodium hydroxide, for example, from about 1 to about 2 weight percent of NaOH.

Although water alone is the preferred liquid medium in which the water-soluble inorganic base is dissolved, it is not essential to use water as the solvent for the base. For example, one may use in lieu of all or part of the water 10 an organic solvent in which the inorganic base is soluble, e.g., a lower alkanol such as methanol, ethanol, and the normal and isomeric forms of propyl through amyl al cohols.

The extraction may be carried out at atmospheric pressure at temperatures up to and including the boiling point of the liquid extract, using a treating vessel provided with a reflux condenser when the maximum temperature of extraction is the reflux temperature. Or, the extraction may be effected under superatmopheric pressure when the temperature of extraction is above the boiling point of the liquid extractant. Depending upon the particular type of extraction vessel employed, the residue undergoing extraction may or may not be agitated during the extraction process.

At the end of the extraction period the crude pulp (i.e., crude delignified material) is separated from the extraction liquor, e.g., by filtration, after which it is washed with a liquid washing medium, more particularly water and, specifically, hot water. After washing, it is mechanically defibrated with water (for instance, in a laboratory with a Sprout-Waldron pre-refiner), after which it is screened to remove the excess water and to provide a uniform product.

In recovering the mercaptan-reacted lignin and thereby purifying the liquid extraction agent, the extraction liquor may be treated with, for example, CO or a dilute mineral acid such as HCl in order to precipitate the ligneous material for recovery. Dialysis also may be employed to separate the lignin from the extraction liquor, Or, the liquor containing the dissolved lignin may be passed through an anion-exchange resin in free-base form thereby selectively to adsorb on the resin anionic materials contained in the liquor while the lignin in purified form passes through the resin for subsequent evaporation of the eluate and recovery of the lignin. This latter technique is more fully described and broadly and specifically claimed in the copending application of William H. Greive and Karel F. Sporck, Ser. No. 418,872, filed Dec. 16, 1964, now abandoned and assigned to the same assignee as the present invention.

Bleaching and/or drying steps are optional depending upon the end-use. If bleaching is to be effected, it is usually done before drying the pulp. Because digestion is carried out in a treating liquor which is at least initially alkaline (and may be alkaline throughout the whole cook ing period), in some cases it may be desirable, prior to the bleaching step, to wash the crude pulp with a dilute aqueous solution of an inorganic acid, e.g., a 5% aqueous HCl solution, thereby to insure a more complete and efficient bleaching action than When bleaching is effected in the absence of such a dilute acid wash. The pulp, with or without further treatment as may be required for the particular end-use, is then suitable for utilization in making any desired cellulosic product including, for example, paper and related products, cellulose acetate, cellulose xanthate, a regenerated cellulose, ethyl celluolse, etc.

One of the advantages of the instant invention is the flexibility with which it lends itself to the obtainment of highor moderate-yield pulps or alpha-cellulose merely by changing such operating parameters as, for example, pH, time, and temperature of digestion, amount of reactive agent, e.g., TGA plus NaSH, in the treating liquor; and the ratio of the components thereof to each other. Furthermore, the method makes possible the recovery of a relatively high proportion of the organic and inorganic by-product material that is left in the treating liquor.

In other words, the operating parameters can be varied to obtain lignin and/or pulp or cellulose residue having the desired properties. The conditions can be varied to produce pulp (hence also paper and paper-like products) having a wide range of physical properties; and, additionally, a recoverable lignin having a wide range of utility.

11 In order that those skilled in the art may better understand how the present invention can be carried into effect, the following examples are given by way of illustration and not by way of limitation. All parts and percentages are by weight unless otherwise stated.

EXAMPLE 1 This examples illustrates the two-stage pulping of a softwood, more particularly isopropanol-extracted Southern pine, the latter being from the area that causes the pine to be sometimes designated as Valdosta pine. The pine was digested with an aqueous treating liquor containing different percentage combinations of an organomercaptan, specifically thioglycolic acid, and an inorganic digestion agent, more particularly NaSH. Sodium hydroxide was used to adjust the initially alkalinity to a pH of 11.0-11.1. Hence the thioglycolic acid was present in the treating liquor in the form of its sodium salt (ie., sodium thioglycolate).

The maximum digestion temperature was 170 C., and the time at that temperature was 2 hours. The liquid extractant used in stage two of the method was a 1% aqueous solution of sodium hydroxide, the extraction temperature was 150 C., and the time at that temperature was 1 hour.

For purpose of comparison with the results obtained using treating liquors containing a combination of NaSH and TGA in the form of its sodium salt, duplicate runs also were carried out wherin NaSH was omitted from the formation.

Additional details are given in Table I-A wherein the runs using the NaSH/TGA combination in the treating liquor are designated as Cook Nos. 1-a and l-b, while those wherein the NaSH was omitted from the liquor are identified as Cook Nos. 1-c and 1d. The apparatus and general procedure used in the runs of the four cooks were substantially the same except in the aforementioned omission of NaSH from the last two cooks. For ease in describing the apparatus and general procedure, Cook No. 1a will be taken as illustrative.

A 600-gram charge (based on the oven-dried weight) of air-dried Southern pine chips from which the isopropanol extractives previously had been removed and a total of 2700 ml. of aqueous liquor at a pH of 11.0 containing 38.4 grams of TGA, 28.2 grams of NaOH (i.e., 11.5 grams NaOH in excess of that required to neutralize the TGA), and 10.8 grams of NaSH were charged into an electrically-heated stationary digester. The miX- ture, at a 4.5-to-1 liquor-to-wood ratio, was heated steadily over a period of one hour to the maximum temperature of 170 C. and maintained thereat under the vapor pressure of the mixture (115-125 p.s.i.g.) for 2 hours. It was then cooled gradually to below 100 C. over a period of 1 hour, after which the charge was removed from the digester and screened to remove the intact, reacted chips from the spent liquor. These chips were washed with hot water to remove the residual cooking liquor, and then drained to remove most of the adhering wash water.

The reacted chips, obtained as above-described, and a 1% aqueous solution of NaOH in a volume (in milliters) corresponding to five times the OD. weight (in grams) of the initial wood used in the initial digestion stage were charged to the same pressure digester used in the digestion step. The mixture was heated steadily to 150 C. over a period of 50 minutes and maintained at said temperature and at near to 63 pounds gauge pressure for 1 hour, after which it was cooled to below 100 C. over a -50 minute period. The resulting pulp was separated from the extraction liquor by filtration, washed with hot water, and mechanically defibrated in a Sprout-Waldon prerefiner to provide a uniform product.

The wet cellulosic pulp obtained as described above was dried to 2030% solids, then made into 8" x 8", 26 lb./MSF basis weight handsheets for testing in the following manner: A minimum of three aliquots of the experimental pulp, inan amount based on the oven-dried weight of the wet pulp, were refined with water at 1% consistency for varying periods of time in a Mead Laboratory Refiner (manufactured by The Bauer Bros. Co., Springfield, Ohio). The degree to which each plup aliquot was refined, as determined by measuring the drainage rate of the pulp in a Slowness Tester (manufactured by Williams Apparatus Co., Watertown, N.Y.) was controlled as much as possible so as to provide three or more refining points bracketing 55 seconds William Slowness. Each of the refined pulp slurries was diluted to 0.5% consistency and uniformly mixed prior to making the handsheets. The handsheets were formed in a 8" x 8 Williams sheet mold from aliquots of the pulp slurry that were measured volumetrically for producing 26 lb. MSF sheets. The pulp consistency on forming the sheets was adjusted to 0.05% by further dilution of the pulp aliquot in the mold. The seven or more sheets (wet webs) formed from each batch of pulp slurry were couched from the wire of the mold onto standard 12" x 12" TAPPI blotters, then stacked between blotters with six blotters separating the sheets. The stack was then pressed for 5 minutes at p.s.i. gauge pressure on a Williams press (manufactured by Wililams Apparatus Company). The pressed sheets, retained on the couch blotters, were dried at 260-280 F. on a steam-heated Noble and wood drier, with the sheet contacting the drum for approximately 2 minutes. Atfer removing the blotters, the dried sheets were conditioned at 50% relative humidity and 73 F. for a minimum of 24 hours before testing.

More detailed information on the cook and extraction conditions and the results obtained are given in Table I-A, II-A and III-A. The value for the pulp characteristic listed as Lignin is the result obtained when the pulp is tested for acid-insoluble lignin using the apparatus and procedure set forth in TAPPI Standard Method T222 m-54. The Williams Slowness test, which measures the rate in seconds per liter at which one liter of plup at 0.3% consistency drains at 20 C. in the Williams Slowness instrument, was used as previously described in order to measure the degree to which the pulp was refined in the Mead refiner. The time in seconds for refining the pulp in the Mead refiner to 55 seconds Williams Slowness, as was determined from the three refinings made, is listed in the column of Table I-A headed Mead Refine Time.

The results of tests on papers (handsheets) made from the pulps produced as described in Tables I-A, II-A and III-A are given in Tables LB, II-B and III-B, respectively. The values for the paper characteristics listed under the columns headed Tensile, Tear, Mullen, Ring Crush and Brightness are the results obtained when the respective handsheets are tested using apparatus and procedures set forth where indicated below:

Density, TAPPI Standard: T411 m-44 Basis Weight, TAPPI Standard: T410 os-61 Tensile, TAPPI Standard: T404 os-61 Tear, TAPPI Standard: T414 ts-64 Mullen, TAPPI Standard: T403 ts-63 Ring Crush, TAPPI Standard: T472 m-51 Brightness, TAPPI Standard: T452 m-58 The only exceptions from the above-identified test methods were that 5 sheets, each 49 square inches in area, were used in the Density and Basis Weight tests.

The tabulated values are adjusted for 26 lb./MSF basis weight paper and for pulp refined to 55 seconds Williams Slowness. The values are determined from the results of tests on the three or four sets of handsheets made from each test pulp as described above.

Tables I-A and I-B follow. The significance of the data set forth in these tables will be commented upon in the portion of this specification that follows Table III-B, at which point the tabulated data relevant to Examples 1 and 2 will be discussed.

TABLE IA.-TWO-STAGE TGA-NaSH PULPING OF ISOPROPANOL-EXTRAOTED SOUTHERN PINEWOOl) First-stage (digestion) conditions Pulp carbo- Time at Second-stage Percent Percent hydrate (per- Cook temp., (extraction) pulp pulp cent of initial No. Chem./100 g. wood pH range Temp, C. hrs. conditions yield lignin O.D. wood) 1-a 6.4 g. TGA, 1.8 g. 11. -7. 1 170 2 1% NaOH, 1 hr. 69. 2 25.8 51.4

N aSH 4.7 g. NaOH. at 150 C. l-b 9.6 g. '1, A, 3.8 g. 11.1-9. 2 170 2 do 64. 5 21.4 50. 7

NaSH, 7.0 g. NaOH. l-c 10iggbIGA, 7.1 g. 11.0-11.7 170 2 do 65.6 26. 2 48. 4

a 1-d 10.0 g. TGA, 7.0 g. 11.05.5 170 2 do 67.0 26.7 49.1

N aOH.

* Duplicate runs with TGA alone are included [or comparison.

TABLE IB.-TESTS ON 26 LBJMSF PAPERS MADE FROM PULPS OF COOKS OF TABLE I-A Papers from Mead refine Williams Density Dry tensile Tear Mullen Ring crush Brightness cook No. time (sec.) slowness (sec.) (p.e.f.) (lb./in.) (g. /16 Sh.) (p.s.i.) (lbs.) (percent) Below are shown the amounts (in grams) of sodium 2-a were the same as were used in Cook No. l-b of thioglycolate (NaTGA), excess NaOH (i.e., the amount Example 1. However, the concentration of the aqueous in excess of that required to neutralize the TGA), and NaOH solution (2% vs. 1% in Example 1) and the the effective alkali (i.e., the excess NaOH calculated as temperature of extraction (100 C. vs. 150 C. in Exam- Na O) for each of Cook Nos. l-a, 1-b, 1-c and 1-d. ple 1) were diflerent. In carrying out the runs with TGA The amounts'of the chemicals per 100 grams O.D. wood used in the treating liquors of each of these cooks are given under the heading Amounts of Chemicals as Charged.

in the absense of NaSH (Cook Nos. 2-b and 2-0), the same extraction conditions were employed as in the processing of the digested chips of Cook No. 2-a.

Tables II-A and II-B follow.

TABLE IIA.-TWO-STAGE TGA-NaSH PULPING OEISOPROPANOL-EXTRACTED SOUTHERN PINEWOOD First-stage (digestion) conditions Pulp carbo- Time at Secondstage Percent Percent hydrate (per- Cook Temp, temp, (extraction) pulp pulp cent of initial N o. Chem./100 g. wood pH range 0. hrs. conditions yield lignin O.D. wood) 2'a 9.6 g. TGA, 8.8 g. 11. 1-9. 2 170 2 2% NaOH, 1 hr. 67. 3 21.0 53. 1

NaSH, 7.0 g. NaOH. at 100 C. 2-b 10. 0 g. TGA, 7,1 g. 11. 0-6. 0 170 2 do 72. 3 25. 2 54. 1

NaOH. 2-@ 21.2 g. TGA, 15.5 g. 11. 0-9.4 170 2 do 67.3 20. 2 53. 7

N aOH.

TABLE IIB.-TESTS ON 26 LBJMSF PAPERS MADE FROM PULPS OF COOKS OF TABLE II-A Papers from Mead refine Williams Density Dry tensile Tear Mullen Ring crus Brightness cook No. time (sec.) slowness (see.) (p.c.i. (lb/in.) (g./16 Sh.) .s.i.) (lbs.) (percent) 152 55. 0 34. 5 48. 8 168 89 60 18. 7 132 56. 0 32. 2 40. 8 166 72 61 19. 6 154 55. 0 33. 0 48. 5 171 91 22. 3

Amounts of chemicals Amounts calculated to (A) The first portion of this example, data on which are given in Tables II-A and II-B, illustrates (as do also Cook Nos. l-c and l-d of Example I and of Tables I-A and I-B) the results obtained when a cook is made with TGA alone (i.e., in the absence of NaSH) as the effective pulping or digestion agent in a two-stage pulping of iso propanol-extracted Southern pine under comparable conditions of digestion and extraction. Or, otherwise stated, the A part of this example illustrates the two-stage pulping of the said Southern pine with the combination of TGA and NaSH in comparison with TGA alone (as do also the cooks of Tables LA and I-B of Example 1).

The same apparatus and general procedures were employed as in Example 1 with the following exceptions. In this section of the example the proportions of chemicals (TGA, NaSH and NaOH) in the run of Cook No.

Below are shown the amounts of chemicals calculated to be in the treating liquors of each of Cook Nos. 2-a, 2-b and 2-c, including the amounts of effective alkali in grams (calculated as Na O), as compared with the amounts of chemicals as initially charged.

(B) The second portion of this example (data on which are given in Tables III-A and III-B illustrates the results obtained when cooks are made using varying proportions of TGA, NaSH and NaOH;- at digestion temperatures varying in maximum temperatures in individual runs from 160 C. to 180 C.; at digestion times of either 1 or 2 hours; and in the extraction stage using a 2% aqueous solution of sodium hydroxide for 1 hour at either or C.

The apparatus and general procedure employed were otherwise the same as were described under Example 1.

15 Tables III-A and III-B follow. The significance of the data set fourth in the aforementioned tables relevant to the A and B portions of this example, as well as that of the data in Tables I-A and I-B, is discussed in the portion of this specification that follows Table IIIB.

16 noted that the process of the invention is much more selective in delignification. The invention provides pulps containing less lignin and having much better properties (as evidenced by the strength characteristics of papers made therefrom) even at materially higher pulp yields.

TABLE III-A.TWO-STAGE TGA-NaSH PULPING OF ISOP RQPANOL-EXTRACTED SO UIHE RN PINEWOOD First-Stage (Digestion) Conditions Pulp carbohy- Second-stage Percent Percent drate (percent Cook pH Temp., Time at (extraction) pulp pulp of initial No. Chem./100 g. wood range C. temp., hrs. conditions yield lignin O.D. wood 3-21. 6.4 g. TGA, 1.8 g. 11. -7. 2 170 2 2% NaOH, 1 hr. 62. 3 21. 4 49. 0

NaSH, 4.7 g. NaOH. at 150 C. 3-b 12.8 g. TGA, 5.1 g. 11. 0-9.3 160 2 do 60. 4 16.8 50. 3

NaSH, 9.2 g. NaOH. 3-0 12.8 g. TGA, 5.1 g. 11.04]. 2 180 1 2% 112.011, 1 hr. 64. 3 21. 5 50. 5

NaSH, 9.3 g. NaOH. at 100 0.

TABLE III-BE-Tests on 26 lbJMSF Papers Made from Pulps oi Cooks of Table III-A Papers from Mead refine Williams Density Dry tensile Tear Mullen Ring crush Brightness cook No. time (sec) slowness (sec.) (p.i.c.) (lb./1n.) (g./16 Sh) (p.s.i.) (lbs.) (percent) 3-a 127 55. 0 32. 5 43. 5 189 87 53 17. 6 3-1) 150 55. 0 34. 3 49. 5 182 98 56 20. 0 3-0 148 55. 0 34. 6 43. 8 183 90 63 18. 3

Below are shown the amounts of chemicals calculated to be in the treating liquors of each of Cook Nos. 3-a, 3-b and 3-0, including the amounts of effective alkali in grams (calculated as Na O), as compared with the amounts of chemicals as initially charged.

Amounts of chemicals Amounts calculated to be as charged in liquor Total Efiective Cool. NaOH, NaSH, NaT GA, NaOH, Alkali, g. No. T GA, g g g. g. g. (as NazO) SIGNIFICANCE OF THE TEST DATA IN TABLES I-A, I-B, II-A, II-B, III-A and III-B (1) Less organomercaptan, specifically TGA, is required for the preparation of high-yield pulp from which can be produced high-strength papers by using a combination of an organomercaptan and an inorganic digestion agent, more particularly a combination of TGA and NaSH, than when TGA is used alone.

(2) The combination of the organomercaptan and inorganic digestion agent, specifically TGA-i-NaSH, is more specific in delignification on than is TGA alone, thereby providing a higher yield of pulp at comparable levels of delignification.

(3) In general, the papers produced from pulps made in accordance with'the invention, specifically by using a treating liquor containing TGA and NaSH, are stronger (especially with respect to Mullen and tensile values) than when TGA is used alone under similar pulping conditions and comparable concentrations of TGA.

(4) Improvements are provided over Kraft pulping, especially with respect to the properties of high-yield pulp. The data show that better properties as to strength of papers, especially as to strength at comparable pulp yields, are obtained from the pulps produced by the process of this invention as compared with papers produced from kraft-process pulp.

(5) The data also show that the invention provides significant improvement over soda-type pulping. When comparisons are made with the data in Tables V-A and V-B involving a one-stage soda cook of pine, it will be The advantages attained by using a treating liquor containing a combination of TGA and NaSH for pulping as compared with one containing TGA alone are evident from a comparison of the data on Cook Nos. l-a and 1-b (TGA-NaSH cooks) with that on Cook Nos. 1-c and l-d (TGA cooks), and the data on papers made from the pulps of these cooks, which data appear in Tables I-A and I-B; and from similar comparisons of the data on Cook No. 2-a (TGA-NaSH cook) with that of Cook Nos. 2-b and 2-c (TGA cooks), and on papers made from the respective cooks, which data are given in Tables II-A and II-B.

Cook No. l-a of Table I provides evidence that, even with less TGA than is used in cooks with TGA alone, delignification is more selective and more complete when the treating liquor contains a combination of TGA and NaSH. Note, too, that a high-yield pulp (69.2% yield) is obtained with properties comparable to the lower yield (65.6 and 67.0%) pulps of Cook Nos. 1c and l-d.

Cook No. 1-b (TGA-NaSH cook) of Tables LA and I-B gave a pulp having better paper properties (especially as to Mullen and tensile values) than that of a pulp wherein TGA alone was used as the digestion agent at comparable concentrations of TGA and under other comparable pulping conditions. Comparisons of the pulp yield and percentages of lignin and carbohydrate in the pulp further show that the TGA-NaSH cook is also more selective in delignification than are the comparable TGA cooks.

Cook No. 2-a (TGA-NaSH cook) of Tables II-A and II-B provides additional evidence of the advantages of using a combination of TGA and NaSH in the treating liquor instead of TGA alone (i.e., in the absence of NaSH). From a comparison of the data on Cook Nos. 2-a and 2-0 it will be noted that good pulping results as to pulp yield, delignification and paper properties (except for brightness) are secured by using TGA-NaSH in combination and wherein the TGA is less than one-half the amount of TGA required to obtain comparable results when TGA is employed alone as the digestion agent in the treating liquor.

When compared with the results of Cook No. 2-b (TGA cook), the data relative to Cook No. 2-a (TGA- NaSH cook) provide evidence that the combination of TGA and NaSH in the treating liquor results in more complete delignification and gives pulps from which can be made stronger papers (especially as to Mullen and tensile strengths) under pulping conditions and TGA concentration comparable to those for TGA Cook No. 2-b.

Comparisons of the TGA-NaSH cooks in Table III, in addition to those of Tables I and II, with the kraft cooks of Table VI, provide evidence that the process of this invention gives a high-yield pulp from which can be made EXAMPLE 3 This example illustrates the pulping (delignification) of isopropanol-ex'tracted hardwood, more particularly black gum wood by the process of this invention.

The apparatus and general procedure employed in the digestion stage with TGA-NaSH and the alkaline-extraction stage of the process were the same are described in Example 1 with the exception that the maximum temperature of digestion was either 150 C. or 170 C.; the time at the maximum temperature was either 1 hour or 2.5 hours; a 1% or a 2% aqueous solution of NaOH was used as the extractant; and the time of extraction was 1 hour at 100 C. instead of 1 hour at 150 C. as in Example 1.

Tables IV-A and IV-B follow.

duced. Also, the data on pulp yield and on the lignin and carbohydrate contents of the pulps show that delignification is much more selective when hardwoods are pulped in accordance with the instant invention as compared with soda pulping. In other words, that a more completely delignified pulp in a high yield can be obtained from hardwood.

EXAMPLE 4 This example illustrates the results of carrying out soda cooks on isopropanol-extracted pine and black gum woods. In the case of the treatment of pinewood (Cook No. 5a) the amount of effective alkali (20.7 g. NaOH) per 100 g. of O.D. wood was 16% calculated as Na O and based on the O.D. weight of the wood. With black gum wood (Cook No. 5-b), however, the amount of effective alkali (10.3 g. NaOH) per 100 g. of O.D. wood was 8% calculated as Na O and also based on the O.D. weight of the wood. In carrying out both cooks, the liquorto-wood ratio was 4:5 to 1, and the digestion time was 39 minutes at a maximum temperature of 170 C.

The soda cooks of this example were made in exactly TABLE IVA.TWO-STAGE TGA-NaSH PULPING OF ISOPROPANOL-EXTRACTED BLACK GUM WOOD First-Stage (Digestion) Conditions Pulp carbo- Time at Second-stage Percent Percent hydrate (per- Cook pH Temp., Temp., (extraction) p u pulp cent of initial N o. Chem./10O g. wood range C. hrs. conditions yield llgnrn O.D. wood) 4-a 6.4 g. TGA, 1.8 g. 11. 0-6. 4 150 2. 5 1% NaOH, 1 hr. 70. 8 23. 0 54. 5

NaSH, 4.6 g. NaOH. at 100 C. 4-b .do 11. 0-6. 4 150 2. 5 2% NaOH, 1 hr. 66. 8 22. 9 51. 5

at 100 C. 4-c 12.8 g. TGA, 5.1 g. 11. 0-9. 6 150 1. 0 do 69. 4 23. 2 53. 3

.NaSH,.9.1 g. NaOH. 4-d. 6.4 g. TGA, 1.8 g. 11. 0-5. 0 170 1.0 do 59. 9 20.8 47. 4

NaSH, 4.7 g. NaOH TABLE IVB.TESTS ON 26 LBJMSF PAPERS MADE FROM PULPS OF COOKS OF TABLE IV-A Williams Paper from Mead refine slowness Density Dry tensile Tear Mullen Ring crush Brightness Cook No. time (sec.) (sec.) (p.c.f.) (lb./in.) (g./l6 Sh.) (p.s.i.) (lbs (percent) Below are shown the amounts of chemicals calculated 7 V the same manner as the kraft cooks hereafter more fully to be in the treating liquors of each of Cook Nos. 4-a,

described in Example 5, the only difference being that 4-b, 4-c and 4-d, including the amounts of effective alkali 45 in the latter case mill Kraft liquor was used while in the in grams (calculated as Na O), as compared with the amounts of chemicals as initially charged.

soda cook of this example the liquor was laboratory-prepared.

TABLE VA.LABORATORY SODA COOKS OF ISOPROPANOL-EXTRACTED BLACK GUM AND PINE WOODS Pulp carbo- Effeetive Percent Percent hydrate (perpH alkali, pulp pulp cent of initial Cook No. Type Wood Chem./100 g. wood range percent Yield Lrgnin O.D. wood) 5-a- Pine 20.7 g. NaOH, 5.0 g. NaQCOa.. 13. 5-13. 3 16.0 62.7 24. 4 47. 4 5-b Black Gum 10.3 g. NaOH 12-9-11. 7 8.0 68. 7 25.0 51. 5

*Calculated as N320 and based on the initial O.D. weight of the wood.

TABLE IVB.TESTS ON 26 LBJMSF PAPERS MADE FROM PULPS OF COOKS OF TABLE V-A Williams Papers from Mead refine slowness Density Dry tensile Tear Mullen Ring crush Brightness Cook No. time (sec.) (sec.) (p.c.f.) lb./1n.) (g./16 Sh.) (p.s.i.) (lbs.) (percent) Additional details on the soda cooks (5a and 5b) Amounts chemicals Ammmtscalculaled be and on papers made from the pulps resulting from the as charged in liquor cooks are given in Tables V-A and V-b above. Total H TGA N OH fil l i l Coo NaOH, NaS Na a a i, g. I No A. g. g: (as NazO) 65 EXAMPLE 5 H 6 4 4 6 L8 L8 L4 This example illustrates two different simulated lab- 4-b:::: 6:4 4.6 1.8 7.9 1.8 oratory kraft cooks of isopropanol-extrated southern ZI"" 32 if; pine. These cooks were made for the purpose of cornparing the properties of the pulps and of handsheets made therefrom with the corresponding products of Ex- The data relative to the pulping of black gum wood by the method of this invention provides evidence that pulps at high yield (approximately 60 to 70%) with much better properties than those obtainable from soda pulping amples 1, 2 and 3. Details of the pulping conditions for the individual cooks (6-a, 6-b, and 6-0) :are given in Table VI-A. The results of tests on papers (handsheets) made from the pulps identified in Table VI-A are given of black gum (Cook No. 5-b of Table V) can be proin Table VI-B.

19 'ORMULATION FOR COOK NOS. 6-a, 6b, and 6-c wood) \ctive alkali as Na O: 35.6 g.p.l. (16.0% on the OD.

wood) Total alkali as Na O: 42.0 g.p.l. (18.9% on the OD.

wood) lulfidity: 33.1% of the active alkali -iquor-to-Wood ratio: 4.5 to 1 Fotal liquor volume: 1800 ml.

The above liquor is prapared by dilution of convenional mill white liquor. The active alkali (A.A.) con- :entration of 16% AA. based on the OD. wood is' the :ame as that of a mill cook. The liquor-to-wood ratio )f 4.5 to 1 differs from the usual 3.4 to 1 ratio of a mill :ook.

Tables VI-A and VI-B follow.

at an 20 different from that suggested by Holmberg in the publication cited in the early part of this specification and whose procedure was there briefly described. It is also separately and patentably distinct from that disclosed and claimed in our copending application Ser. No. 605,978 and in our 00- pending application Ser. No. 605,957. The first of these copending applications is concerned with the delignification of lignocellulosic material by a two-stage treatment wherein the reactive agent in the first-stage treating liquor,

al pH not exceeding 12.0, is an organomercaptan. Thereafter a second-stage treatment is applied whereby the organomercaptan-reacted lignin retained by the digested lignocellulosic material is extracted by contacting the residue with an extractive amidogen compound, e.g.,

an alkanolamine, aniline or urea. The second of these copending applications is concerned With a one-stage delignification of lignocellulosic material by treatment with a treating liquor containing a specified minimum amount of an organomercaptan, and which also contains certain specified minimum amounts of effective alkali and of total alkali.

The present invention is also separately and patentably distinct from the inventions disclosed and claimed in the copending applications of Carl A. Johnson, Ser. No.

TABLE VIA.-LABORATORY KRAFT COOKS OF PINEWOOD AT 16% ACTIVE ALKALI Cook conditions Pulp data Effective Alkali Pulp carbo on Wood Time at Pulp hydrate (per Sulfidity (percent as pH Temp. temp. Liquor: Yield lignin cent of initia look No. Chem./100 g. wood (percent) N540) range 0.) (min.) wood (percent) (percent) O.D wood) i-a 6.7 g. Na2S, 13.8 g. 33. 1 13.4 13. 5-13. 1 170 39 4. 5-1 55. 9 13. 4 48. 4

NaOH, 5.0 g. NazC 0 5-D do 33. 1 13. 4 13. 5-13. 3 170 12 4. 5-1 60. 8 18. 9 9. 3 5-0 ..do.. 33. 1 13. 4 13. 5-13. 3 170 4 4. 5-1 64. 3 20. 6 51. 1

*Oalculated as NazO and based on the OD. weight of the wood.

TABLE IVB.-TESTS ON 26 LB .IMSF PAPERS MADE FROM PULPS OF COOKS OF TABLE VI-A Williams I Papers from Mead refine slowness Density Dry tensile Tear Mullen Ring crush Brightness Cook No. time (sec.) (sec.) (p.e.i.) lhJin.) (g./16 Sh.) (p.s.i.) (lbs.) (percent) 51% 153 55. 0 33. 53. 9 197 109 70. 0 18. 8 64) 205 55. 0 30. 1 44. 3 177 90 65. 0 17. 8 [5-0 217 55. 0 30. 2 42. 5 178 81 60. O 18. 7

From the foregoing description it will be seen that the present invention provides a two-stage method of delignifying (pulping) lignocellulosic material that provides distinct advantages as compared with the use of an organomercaptan :alone as the reactive component of a treating liquor; and, also, as compared with delignification by conventional soda or kraft cooking or digestion techniques. The method includes the step of: (A) Digesting the lignocellulosic material with a treating liquor containing a reactive agent that is reactive with the lignocellulosic material. The reactive agent consists essentially of (a) at least one organomercaptan and (b) at least one inorganic digestion aid. Numerous examples of the aforementioned components of the reactive agent have been given hereinbefore. The treating liquor preferably has an initial, alkaline pH not exceeding about 12.0. The reactive agent is present in the treating liquor in an amount corresponding to at least 2.5, preferably from about 5 to about 50, and still more preferably from about 5 to about weight percent based on the weight of the oven-dried lignocellulosic material. In all cases the temperature and time of digestion are sufficient to convert the lignocellulosic material to a treated material containing mercaptan-reacted lignin. The method of this invention also includes the step of: (B) Extracting the mercaptan-reacted lignin retained by the digested lignocellulosic material by contacting it with a dilute solution of a water-soluble inorganic 'base, numerous examples of which previously have been given.

From the foregoing description of the instant invention it will be further seen that it is materially and unobviously 606,024 and Ser. No. 606,012. The first-identified Johnson application is concerned with the use of a combination of an organomercaptan and a hydrotrope agent in a method for pulping (digesting) lignocellulosic material. The latter Johnson application is concerned with a particular twostage treatment for delignifying lignocellulosic material wherein the reactive agent in the first-stage treating liquor is an organomercaptan; that is, a treating liquor containing no inorganic digestion agent such as the monoor disodium sulfides that are used in practicing the present in- P vention. In the second stage of this Johnson method, the

organomercaptan-reacted lignin retained by the digested lignocellulosic material is extracted by contacting it with a dilute solution of a water-soluble inorganic base.

All of the aforementioned copending applications, filed concurrently herewith, are assigned to the same assignee as the present invention.

We claim:

1. The method of pulping lignocellulosic material by a two-stage treatment which includes the steps of (A) digesting said material with a treating liquor containing a reactive agent that is reactive with the said lignocellulosic material and which consists essentially of (a) at least one organomercaptan and (b) at least one inorganic digestion aid, said reactive agent being present in the treating liquor in an amount corresponding to at least 2.5 weight percent based on the weight of the oven-dried lignocellulosic material, said treating liquor having an initial, alkaline pH not exceeding about 12.0, the temperature and time of digestion being suflicient to convert the lignocellu- 21 losic material to a treated material containing mercaptan-reacted lignin; and

(B) extracting mercaptan-reacted lignin retained by the digested lignocellulosic material by contacting it with a dilute solution of a water-soluble inorganic base.

2. A method as in claim 1 wherein the reactive agent consists essentially of, by weight, (a) a major amount of an organomercaptan and (b) a minor amount of at least one inorganic digestion aid selected from the group consisting of the alkali-metal hydrosulfides, monosulfides, polysulfides, sulfites, bisulfites and borohydrides.

3. A method as in claim 2 wherein the inorganic digestion aid is sodium hydrosulfide.

4. The method as in claim 2 wherein the organomercaptan is thioglycolic acid.

5. The method as in claim 2 wherein the organomercaptan is thioglycolic acid and the inorganic digestion aid is sodium hydrosulfide.

6. The method as in claim 1 wherein the lignocellulosic material includes softwood.

7. The method as in claim 1 wherein the lignocellulosic material includes hardwood.

8. The method as in claim 1 which includes the step of removing the excess liquor from the treated material from Step A; and the mercaptan-reacted ligning retained by the digested lignocellulosic material, and from which the excess liquor has been removed, is extracted with a dilute aqueous solution of an alkali-metal hydroxide or carbonate.

9. The method as in claim 8 which includes the step of washing the residue that remains after removing the excess liquor from the mercaptan-treated lignocellulosic material; and the mercaptan-reacted lignin in the washed residue is extracted with a dilute aqueous solution of sodium hydroxide.

10. The method of pulping lignocellulosic material by a two-stage treatment which includes the steps of (A) digesting said material With a treating liquor containing a reactive agent that is reactive with the said lignocellulosic material and which consists essentially of (a) an organomercaptan and (b) sodium hydrosulfide,

said reactive agent being present in the treating liquor in an amount corresponding to from about to about 50 weight percent based on the weight of the oven-dried lignocellulosic material, said sodium hydrosulfide constituting from about 10 to about 40 weight percent of the total amount of the (a) and (b) components of the reactive agent, said organomercaptan being one represented by the general formula wherein Z represents an alkylene radical containing from 1 through 10 carbon atoms; R represents a number of the group consisting of (a) hydrogen, (b) alkyl radicals containing not more than 10 carbon atoms, and (c) a saltforming cation; and n represents an integer from 1 up to that of the combining power of the alkylene radical represented by Z,

said treating liquor having an initial, alkaline pH not exceeding about 12.0, and the temperature and time of digestion being sufiicient to convert the lignocellulosic material to a treated 'material containing mercaptan-reacted lignin; (B) removing the excess liquor from the treated material from Step A; and (C) extracting mercaptan-reacted lignin retained by the digested lignocellulosic material, and from which the excess liquor has been removed, by contacting it with a dilute aqueous solution of sodium hydroxide. 11. The method as in claim 10* wherein the organomercaptan is one represented by the general formula HS(CH -COOR wherein n represents an integer from 1 to 8, inclusive, and R represents a member of the group consisting of (a) hydrogen, (b) alkyl radicals containing not more than 8 carbon atoms, and (c) a salt-forming cation.

12 The method as in claim 11 wherein the organomercaptan is thioglycolic acid.

13'. The method as in claim 10' wherein the organomercaptan comprises thioglycolic acid and the dilute aqueous solution of sodium hydroxide used in Step C contains from 0.5 to 5 weight percent of NaOH.

14. The method as in claim 10* wherein the organomercaptan comprises thioglycolic acid; the treating liquor has an initial pH within the range of from about 9 to about 11; the digestion temperature is within the range of from C. to 200 C.; and the dilute aqueous solution of sodium hydroxide is one that contains from 1 to 2 Weight percent of NaOH.

15. The method as in claim 10 wherein the organomercaptan includes thioglycolic acid; the reactive agent is present in the treating liquor in an amount corresponding to from about 5 to about 25 Weight percent based on the Weight of the oven-dried lignocellulosic material; the treating liquor has an initial pH within the range of from about 9 to about 11; the digestion temperature is within the range of from about to about C.; the digestion time within the aforesaid temperature range is from about /2 to about 3 hours; and the dilute aqueous solution of sodium hydroxide is one that cOntains from 1 to 2 weight percent of NaOH.

16. The method as in claim 15 which includes the step of water-washing the residue that remains after removing the excess liquor from the lignocellulosic material that has been digested in the defined treating liquor.

17. A mercaptan-reacted lignin that is the product of the process defined in claim 1.

References Cited Wood Chemistry, Wise & John, 2nd ed., vol. I, pub. by Reinhold Pub. Corp, New York, N.Y., 1952, p. 435 and p. 498.

HOWARD R. CAINE, Primary Examiner US. Cl. X.R.

22 3 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 5, 9 ,99 Dated January 20, 1970 Inventor(s) William E. Fisher and Shiblev A. Hider It is certified that error appears in the above-identified patent and that-said Letters Patent are hereby corrected as shown below:

Column 1, line 65, "dop" should be --drop--. Column 7, line 18 "loquor" should be --liquor- Column 8, line 18, "panel" should be --panol- Column 9, line 9, "pertient" should be --pertinent Column 10, line 59, "Sporck" should be -SporeK-. Column 1%, line 26, "absense should be --absence-- Column 18, "Table Iv-B" should be --Table v-B--; line 67, "extrated" should be --extraoted--. Column 19, line 18, "prepared" should be --prepared--; lin 55, "The" should be --This--. Column 21, line 26, "ligning" should be --lig;nin--; line 58, "number" should be --member--.

Signed and sealed this 30th day of November 1971.

(SEAL) Attest:

EDWARD M.FLETCHEH,JR. ROBERT GOTTSCHALK Attesting Officer Acting Commissioner of Patents 

